The mobile communication system, as shown in FIG. 1, is configured of a gateway device 1, a plurality of base stations 2 to 4, and a mobile station 8. Each of the base stations 2 to 4, which constitutes one or a plurality of cells 5 to 7, makes communication with the mobile station being positioned in the above cell. The gateway device 1, which is connected to each of the base stations 2 to 4, and is connected to a high-rank network as well not shown in the figure, controls communication between the high-rank network and the mobile station existing in the cell of the base station that is under control thereof, and transfers information.
As such a system, for example, there exists E-UTRAN (Evolved Universal Terrestrial Radio Access Network), of which activity on standardization is ongoing, and its outline is described in Non-patent document 1.
In such a mobile communication system, a service area is divided into a plurality of position registration areas, and a plurality of the base stations 2 to 4 existing in each position registration area notify peculiar position registration area identification information. The mobile station 8 receives and stores the above identification information, and compares it with identification information that has been periodically received and stored. And, the mobile station 8, when the received identification information differs from the stored identification information, notifies the position registration information to the gateway device 1 via the base stations 2 to 4.
With this, when a terminal (not shown in the figure) connected to the high-rank network commences communication with the mobile station 8, the gateway device 1 cannot specify the base station with which the mobile station 8 is communicable, but can specify a position registration area 9 in which the mobile station 8 exists, whereby the gateway device 1 transmits paging information (calling information) of the mobile station 8 to each of the base stations 2 to 4 being included in the position registration area 9, and in addition, the base stations 2 to 4 transmits the paging information by employing a paging channel in cells 5 to 7, respectively.
Each of the base stations 2 to 4 transmits a pilot channel at a predetermined power, so the mobile station 8 measures reception powers of the pilot channels of the surrounding base stations 2 to 4 at a predetermined period, and receives the paging channel being transmitted by the base station of which the above reception power is maximum.
In FIG. 1, the mobile station 8 receives the paging channel of the base station 2, thereby to receive the paging information being transmitted from the base station 2, and transmits a response signal to the base station 2 because it exits in the cell 5 of the base station 2, so the transmission power of the pilot channel of the base station 2 is maximum. And, the base station 2 notifies the gateway device 1 that communication with the mobile station 8 is enabled in the base station 2. With this, the gateway device 1 connects the terminal that is going to commence the communication with the mobile station 8, and the mobile station 8 via the base station 2, thereby allowing the communication to be commenced.
This system necessitates setting the transmission power of the paging channel so that the paging information can be received at a reception error ratio within a desired range over the entirety of the cell for which each base station provides a communication service, and yet no excessive interference is given to the circumference. Such a transmission power differs depending upon a position relation with the base stations in the circumference of each base station, and a propagation environment. For this, the method is thinkable of calculating a probability (a reception error ratio) that when the base station transmits paging information to the mobile station, the mobile station fails in receiving the above information, and changing the transmission power so that the above reception error ratio falls under a certain range.
As shown in FIG. 9, when the mobile station fails in receiving the paging information, the gateway device retransmits the paging information after a lapse of a certain time. And, the method is thinkable in which the gateway device, when having received a response to the retransmission, determines that the reception error has occurred in a mobile station, increase the transmission power of the paging channel in the above base station, and when no reception error occurs, decreases the transmission power of the paging channel.
While, in this method, the gateway device controls the transmission power of the paging channel of each base station, it is desirable that the gateway device does not take a control associated with radio transmission between the base station and the mobile station, and the base station entirely takes such a control radio.
However, when, notwithstanding the transmission of the paging information, the base station was not able to receive its response, it cannot made a determination as to whether the cause is that the accessed mobile station did not exist within the cell of the above base station, or that the mobile station failed in receiving the paging information because a reception quality of the paging channel was not good even though the accessed mobile station existed within the cell of the above base station.
Thereupon, in Non-patent document 2, a scheme is described for enabling the base station to identify whether the response to the transmission of the paging information is a response to the first transmission or a response to the retransmission so that the base station can control the transmission power of the paging channel. In this scheme, as shown in FIG. 10, the gateway device affixes RSN (Retransmission Sequence Number) to the paging information, and transmits it. RSN=0 with the first transmission, and RSN=1 with the first retransmission. And, the base station as well transmits to the mobile station the paging information with this retransmission sequence number affixed, and this retransmission sequence number is affixed to the response as well by the mobile station. With this, from a number of times N1 of responses to the first transmission of the paging information, and a number of times N2 that the responses are received due to the retransmission, the base station approximately can estimate magnitude of the reception error ratio of the paging information in the mobile station within the cell with N2/(N1+N2), and hence can optimize the transmission power of the paging channel.    Non-Patent document 1: 3GPP TS 36.300 V8 (2007-03), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Release 8)    Non-patent document 2: R2-071532 (26-30 Mar. 2007), Initial Standardisation Requirements from Self-Organizing Networks